This application claims priority of Japanese patent Application No. 2000-205991, filed on Jul. 7, 2000, and Japanese patent Application No. 2001-132083, filed on Apr. 27, 2001.
This invention relates to an optical gate and an optical phase modulator, and more specifically relates to an optical gate applicable to optical transmission systems, optical network systems and optical switching systems and to an optical phase modulator useful for realizing such an optical gate.
To realize an ultra large capacity and ultra high speed optical communication system, it is important to obtain an optical gate element capable of switching ON/OFF of an optical signal using the light. Especially, when a signal transmission rate per wavelength becomes as fast as 10 Gb/s or more, it is difficult to process a signal electrically in terms of the operation speed and the energy consumption. Accordingly, an optical gate or an optical switch to directly turn ON/OFF or to switch the optical signal using another optical signal has enthusiastically developed.
There are several types of conventional optical switches, for example, one using cross gain modulation (XGM) in a medium whose gain nonlinearly varies according to the intensity of input light such as a semiconductor optical amplifier (SOA), one using cross phase modulation (XPM) in a medium whose refractive index nonlinearly varies according to the intensity of input light such as a SOA, and the one using four wave mixing (FWM).
The response speed of the optical switch using the FWM is extremely high. However, it has a demerit to need high power for the ON/OFF of the light because its conversion efficiency is small and its wavelength dependency is large.
The optical switch using the XGM or XPM has a large switching efficiency because both XGM and XPM utilize the phenomena based on the process causing the pumping of actual carriers. It is reported that in the XPM, by switching ON/OFF of the optical signal by a control light pulse in an interference system, the operation speed of 10 Gb/s or more can be realized. However, since the standard XPM utilizes interferometers having two light paths such as Mach-Zehnder interferometers or Michelson interferometers, the circuit tends to be complicated. Furthermore, it is difficult to adjust the operation conditions of the two SOAs in a necessary perfect balance.
As means to solve the above problems in the XPM, having proposed is an optical switch to have a polarization division interferometer circuit configuration which physically has a single light path by dividing an optical signal into two orthogonal polarization components using a birefringent medium, and combining the two orthogonal polarization components again after passing them through the SOA of the nonlinear medium (for example, see N. S. Patel et al. Optics Letters, vol. 21, pp. 1466-1468, 1996). This optical switch is called as Ultrafast Nonlinear Interferometer (UNI).
FIG. 7 shows a schematic diagram of the optical switch disclosed in the above-mentioned paper. An optical signal 212 of wavelength 1550 nm enters an optical signal input port 210, and a control light 216 of wavelength 1540 nm enters a control light input port 214. The optical signal 212 contains, for example, a 40 Gbit/s optical clock signal of linear polarization, and the control light 216 contains a 40 Gbit/s optical RZ pulse train synchronized with the optical signal 212.
The optical signal output 212 from the optical signal input port 210 enters a 7.5 m long polarization preserving fiber 218 at an angle of 45xc2x0 of the polarization plane relative to the birefringent axis of the fiber. The polarization preserving fiber 218 functions as a birefringent medium to divide the input optical signal into two polarization components and to output them after separating them in the time base by the amount (12.5 ps) of the polarization mode dispersion of the polarization preserving fiber 218. A WDM optical coupler 220 combines the output from the polarization preservation fiber 218 with the control light 216 from the control light input port 214. The timing between the optical signal 212 and the control light 216 is adjusted so that at the output stage of the WDM optical coupler 220, a control light pulse 226 is located between a preceding optical signal pulse 222 and a following optical signal pulse 224 output from the polarization preserving fiber 218. The preceding optical signal pulse 222, the control light pulse 226, and the following optical signal pulse 224 enter a semiconductor optical amplifier (SOA) 228 in this order.
The SOA 228 is forward biased by a direct power source 230. For example, the SOA 228 consists of a buried waveguide using the InGaAsP/InP system as an active layer material, and both ends are applied with antireflection coating. When the control light pulse 226 enters, the gain in the SOA 228 instantly decreases due to the stimulated emission, gain saturation occurs, and the carrier density in the SOA 228 decreases. Since the refractive index of the semiconductor depends on the carrier density of the inside (band filing effect), the refractive index variation (which results in XPM) occurs at this point. That is, the refractive index of the SOA 228 varies before and after the entry of the control light pulse 226. Therefore, the following optical signal pulse 224 receives a phase shift different from that of the preceding optical signal pulse 222 while transmitting in the SOA 228. Since the amount of the phase shift varies according to the optical intensity and wavelength of the control light pulse 226 and injected electric current of the SOA 228, the optical intensity and wavelength of the control light pulse 226 and the injected current of the SOA 228 are set so that the amount of the phase variation of the following optical signal pulse 224 caused by the existence and the nonexistence of the control light pulse 226 becomes xcfx80. With this configuration, the phase of the following optical signal pulse 224 output from the SOA 228 differs by xcfx80 according to whether or not the control light pulse 226 exists.
The optical signal pulses 222 and 224 passed through the SOA 228 enter a 7.5 m long polarization preserving fiber 232 in the direction that the polarization plane of the preceding pulse 222 coincides with the slow axis of the polarization preserving fiber 232 and the polarization plane of the following pulse 224 coincides with the fast axis of the fiber 232. With this configuration, the time difference between the optical signal pulses 222 and 224 is almost disappeared after they passed through the polarization preserving fiber 232. To cancel the individual difference of the polarization mode dispersion amount between the polarization preserving fibers 218 and 232, a polarization phase adjuster 234 is disposed at the output of the polarization preserving fiber 232. Reference numerals 224a and 226a denote the preceding optical signal pulse and the following optical signal pulse output from the polarization phase adjuster 234 respectively.
A polarizer 236 is disposed on the output side of the polarization phase adjuster 234 so that the polarizer 236 passes the light having the same polarization direction with that of the composite polarization of the preceding optical signal pulse 222a and the following optical signal pulse 224a when the optical signal pulse 226 exists. When the optical signal pulse 226 does not exist, the composite polarization direction of the preceding optical signal pulse 222a and the following optical signal pulse 224a becomes orthogonal to that of the polarizer 236. Accordingly, the polarizer 236 passes only the optical signal in the condition that the control light pulse 226 exists out of the optical output from the polarization phase adjuster 234. An optical bandpass filter 238 exclusively extracts the component having the wavelength equal to that of the optical input signal 212 from the output of the polarizer 236. With this operation, the remaining components of the control light 216 are removed.
In the conventional example shown in FIG. 7, the operation of the optical switch is realized using the XPM of the SOA 228. Since the signals passed through the sole SOA 228 are interfered each other, it is unnecessary to completely balance the operating conditions of two SOAs unlike an interferometer having two optical paths, and accordingly the operation becomes stable. Moreover, its speed is so high that an optical switch as fast as 100 Gbit/s or more has been reported (see K. L. Hall et al., Optics Letters, vol. 23, pp. 1271-1273, 1998).
However, the recovery time of the XPM in the SOA 228 is as long as approximately 100 ps. Accordingly, although the optical switching speed itself is very high, the pattern effect becomes evident. For example, the characteristics to switch the optical signal 212 vary per pulse when applied to the control light 216 of the random pulse train.
FIGS. 8(a)-(d) show waveform examples showing the pattern effect, and the time variation of the carrier density in the SOA 228. FIG. 8(a) shows the waveform of the input optical signal 212, FIG. 8(b) shows the waveform of the control light 216, FIG. 8(c) shows the waveform of the optical signal output from the optical bandpass filter 238, and FIG. 8(d) shows the time variation of the carrier density. As shown in FIGS. 8(a), (b), each pulse of the input optical signal 212 has a uniform pulse height, and each pulse of the control light 216 also has a uniform pulse height. When the control light pulses continue, the phase shift amount of the SOA 228 reduces because the XPM is repeated without achieving the complete recovery. Accordingly, when the control light pulses continue, the peak value of the optical output signal from the optical bandpass filter 238 reduces as shown in FIG. 8(c). This phenomenon is called the pattern effect. As shown in FIG. 8(d), the amount of the carrier variation caused by the input of the control light pulse, namely the amount of the refractive index variation, differs per control light pulse. This causes the pattern effect.
Also, in the conventional example, since the interferometer is composed of polarization dividing/combining using the birefringent of the two several meter long polarization preserving fibers 218 and 232, there is a problem in which the operation tends to be unstable because it is easily affected by the ambient conditions, such as temperature and vibration. In addition, it is not suitable for the mass production due to the structure.
Furthermore, in the high speed optical transmission systems, it has been expected to realize an optical TDM demultiplexer which not only extract a pulse component of specific time slot from an optical pulse train but also output the remaining time slot pulse component.
It is therefore an object of the present invention to provide an optical gate to shorten the recovery time of XPM in a SOA and to operate with stability and less pattern effect.
Another object of the present invention is to provide an optical gate which is compact, outstanding at the stability against long term environmental variation, and suitable for the mass production.
A further object of the present invention is to provide an optical gate capable of demultiplexing an optical input pulse train into two portions using optical control in the time domain.
Still a further object of the present invention is to provide an optical phase modulator of high-speed operation to modulate a phase of an optical signal using another light.
An even further object of the present is to provide an optical phase modulator to stably operate at high speed.
An optical gate according to the invention consists of a polarization divider to divide an optical signal into two orthogonal polarization components and to output them as a first polarization component which precedes in the time base and a second polarization component which follows the first one in the time base, a semiconductor optical amplifier to modulate a phase of the second polarization component output from the polarization divider according to a control light, an assist light supplier to supply to the semiconductor optical amplifier an assist light to help the recovery of the refractive index variation of the semiconductor optical amplifier caused by the control light, a polarization combiner to combine the first and second polarization components of the optical signal transmitting the semiconductor optical amplifier so as to adjust them in the same time location, and a polarization extractor to extract a predetermined polarization direction component from the output from the polarization divider.
An optical phase modulator according to the invention consists of a semiconductor optical amplifier to which a control light and optical signal input and which is forward biased and varies its refractive index relative to the optical signal according to the intensity variation of the control light, and an assist light supplier to supply to the semiconductor optical amplifier an assist light to help the recovery of the refractive index variation of the semiconductor optical amplifier caused by the control light.